Two men are walking along a horizontal straight line in the same direction. The man in front walks at a speed $1.0 \mathrm{~ms}^{-1}$ and the man behind walks at a speed $2.0 \mathrm{~ms}^{-1}$. A third man is standing at a height 12 m above the same horizontal line such that all three men are in a vertical plane. The two walking men are blowing identical whistles which emit a sound of frequency 1430 Hz . The speed of sound in air is $330 \mathrm{~ms}^{-1}$. At the instant, when the moving men are $10 m$ apart, the stationary man is equidistant from them. The frequency of beats in Hz , heard by the stationary man at this instant, is $\_\_\_\_$。